As long as a Deathbringer isn't completely motionless from the start, it's actually a fun boss. You can stun him with shock bombs/arrows and afterwards his weakspots will be exposed for a couple of seconds. If you blow off a weakspot of his legs it will fall over and give you another opportunity to hit a few weakspots.
It's possible to freeze the Deathbringer, but in my experience freezing him doesn't last longer than stunning him.
Playlist of my HZD Very Hard run: • Horizon Zero Dawn - The Point of the Spear...